Some of conventional driving apparatuses adopt a position sensorless method so designed as to estimate the rotor position of a brushless DC motor and thereby obtain the commutation time point of the motor and drive it. If a trouble occurs in such a driving apparatus or load fluctuation occurs, it can be brought into a state of loss of synchronism in which it cannot drive the motor as intended any more.
JP-A-2004-104935 discloses a technology for, when it is detected that a motor has been brought into a state of loss of synchronism and has stopped, resuming drive control on the motor. However, according to this technique, in case of a motor or the like for driving an electric vehicle, it is inappropriate to stop the rotation of the motor while the vehicle is traveling even though it has been brought into a state of loss of synchronism. The rotation of the motor must be maintained as much as possible. In this technique, after it is detected that a motor has completely lost synchronism, the loss of synchronism is coped with.
JP 4-317587A, U.S. Pat. No. 5,432,414 (JP 5-284781A) and JP 7-327390A disclose technologies for starting the motor by varying a frequency of excitation (energization) of the motor. Those technologies are proposed, because a motor is temporarily rotated in a reverse direction or torque produced in a motor is too large, and this causes over-speed and loss of synchronism, resulting in a lengthened starting time in the conventional apparatus. However, the proposed circuit for varying an excitation frequency is complicated, and this inevitably increases the size of the circuit. For example, when an excitation frequency is varied by digital processing, the number of bits of a counter for counting cycles corresponding to a frequency is increased.
JP 11-18478A disclose a technology to detect a time point at which an electrical angle of a motor becomes equal to a predetermined electrical angle based on an induced voltage developed as a terminal voltage of the motor. According to this technology, limitation is imposed on a permitted period for which detection of a time when the predetermined electrical angle occurs is permitted. However, a detected value of the rotational speed of a motor transitions to too high a value or too low a value and is fixed there. In these cases, it is difficult to control the rotating state of the motor as desired. When it transitions to too high a value or too low a value, a time when a predetermined electrical angle occurs does not fall within the permitted period. There are cases where, for example, power supply voltage or the load on a motor abruptly fluctuates and this causes the rotational speed of the motor to abruptly fluctuate. Also, in these cases, a time when a predetermined electrical angle occurs may temporarily fall outside the permitted period. For this reason, if, when a time when a predetermined electrical angle occurs does not fall within a permitted period, the rotating state is determined to be abnormal. There is a possibility that both a state (loss of synchronism state) in which it is difficult to control the rotating state as desired and a temporary rotational fluctuation state caused by load variation or the like are determined to be abnormal. It is thus difficult, for example, to continuously control a rotary machine if only load variation occurs.
US 2005/0258788 (JP 2005-333689A) discloses determination of an electrical angle of a motor by detecting induced voltages, that is, terminal voltages. When a three-phase motor is started, all switching elements of an inverter are OFF, and thus each phase of the three-phase motor is in a high-impedance state. For this reason, a situation in which a neutral point voltage is equal to the potential of each phase of the three-phase motor can occur. If noise is mixed when the induced voltage is detected in this state, the neutral point voltage and the voltage of each phase frequently cross each other. Eventually, the zero-crossing time is frequently erroneously detected. For this reason, for example, a system required to operate an inverter based on a detection signal with respect to zero-crossing time point from immediately after start of a three-phase motor cannot appropriately meet this requirement.
Further, in US 2005/0258788, the time required for the rotor to rotate by a predetermined interval of electrical angle is determined from time intervals between occurrences of time point with which the above zero-crossing occurs. Time point with which the time required passes after an occurrence of zero-crossing time point is taken as specified time point with which an angle that provides a basis for switching operation occurs. When a specified time point is set by the above method when the three-phase motor is started, the specified time point is set by shortening the predetermined interval of electrical angle used in the above computation of the time required. If this time point is calculated in the initial stage of startup as under normal conditions, this time point is unexceptionally delayed from a time point with which a reference angle occurs. In this case, the specified time point is set by determining a time required from an occurrence of zero-crossing time point to when a reference angle occurs based on a time interval between occurrences of the zero-crossing time point. The inventors found that, to make this setting with accuracy, the rotational speed of the motor must be stable. For this reason, the time point with which the reference angle occurs cannot be set with accuracy not only when the motor is started but also generally when the rotational speed largely fluctuates. This can lead to degraded controllability of the motor.
JP 2642357B1 discloses an example of a conventional control apparatus for multi-phase rotary machines. In another technique for controlling a rotary machine (three-phase brushless motor), a 120°-energization method illustrated in FIG. 50 is proposed. In this figure, (a) illustrates the transition of terminal voltages Vu, Vv, Vw; (b) illustrates the transition of comparison signals PU, PV, PW as a result of comparisons of the terminal voltages Vu, Vv, Vw indicated by solid lines in (a) with a reference voltage Vref; (c) illustrates the transitions of a one-bit combined signal PS obtained by logically combining the comparison signals PU, PV, PW; and (d) illustrates the transition of a detection signal Qs obtained by shaping the waveform of the combined signal PS. With time point (zero-crossing time point) with which the terminal voltages Vu, Vv, Vw indicated by (a) agree with the reference voltage Vref, the output of the comparison signals PU, PV, PW is inverted. In reality, however, the output of the comparison signals PU, PV, PW is also inverted when the operation of the switching elements of an inverter (power conversion circuit) connected with the brushless motor is changed. This inversion is caused by the passage of a current through diodes connected in parallel with the switching elements. For this reason, the rising edges and the falling edges of the combined signal PS obtained by logically synthesizing the comparison signals PU, PV, PW coincide with not only zero-crossing time point. Some of them coincide with time point with which a current is supplied though the diodes. Meanwhile, all the rising edges and the falling edges of the detection signal Qs obtained as a result of waveform shaping coincide with zero-crossing time point.
The electrical angle of the brushless motor is uniquely determined by zero-crossing time point. For this reason, the following can be implemented by changing the operating state of switching elements at the time (specified time point) when a time required for a motor to rotate by a predetermined angular interval (e.g., 30°) from the zero-crossing time has passed. The brushless motor can be controlled by a 120°-energization method. More specifically, a time-series pattern with respect to the operation of switching elements is predetermined. Therefore, control by the 120°-energization method can be achieved by operating the switching elements according to the above pattern each time the specified time point occurs.
Since the detection signal Qs is a one-bit signal, it is impossible to discriminate one zero-crossing time from another in the three-phase brushless motor according to the signal. For this reason, if the rotating state of a brushless motor becomes abnormal or noise is mixed in a terminal voltage Vu, Vv, Vw or the like, there is a possibility that the controllability of the brushless motor is significantly degraded. More specific description will be given. Even if the brushless motor is rotated in reverse, for example, it is difficult to detect this reverse rotation from the detection signal Qs. Therefore, there is a possibility that change of the operation of the switching elements when a time required from a rising edge or a falling edge of the detection signal Qs has passed (specified time point) is continued as under normal conditions. In this case, the brushless motor cannot be appropriately controlled.
There is known a technique for carrying out the following for the purpose of controlling the output of the brushless motor, controlling and limiting a current supplied to the brushless motor, or for other like purposes. During a permitted period for the on operation of switching elements, defined based on the above specified time point, PWM modulation processing is carried out to repeatedly turn on and off the switching elements. In this case, however, a problem arises. In PWM modulation processing, switching elements are frequently switched from ON state to OFF state, and a current is thereby frequently passed through diodes. Eventually, the comparison signals PU, PV, PW and the combined signal PS are frequently inverted. At this time, it is difficult to generate the detection signal Qs as an appropriate signal synchronized with zero-crossing time point. Therefore, it is difficult to appropriately set a specified time point.